"cache" essentially means possibly available for faster access.
What faster access is anybodies guess here.
e.g. A disk or SD card accces might cache into memory.
Instructions could be fetched a page at time as other stuff is going on and lets call this L1 Cache for the sake of argument. Thus the next instruction comes from L1 cache which is INSIDE the processor.
You want to use cache for frequently accessed stuff. By "cascheable" they may mean you can place it there.